A. Field of the Invention
The invention relates to a brake band device, and particularly a brake band device for selectively braking rotation of a cylindrical clutch drum within an automatic transmission.
B. Description of the Background Art
An automatic transmission is generally provided with a plurality of clutch devices and brake band devices for automatic gear changes. These clutch devices and brake band devices are used in a wet environment, i.e., in the presence of lubricating oil.
One such brake band device typically includes a strap that extends around a clutch drum forming one clutch device. The strap is a band-like member having a substantially circular form. A lining is fixed to the inner peripheral surface of the strap and a pair of brackets fixed to or formed on at opposite ends of an outer surface of the strap.
In the brake band device, the lining is made of a friction material that is pressed against the outer peripheral surface of the clutch drum by moving the paired brackets toward each other thereby tightening the brake around the rotating clutch drum. Contact between the lining and the outer surface of the clutch drum causes the clutch drum to be braked, ceasing rotation. When the clutch drum stops rotating, the clutch device enters an engaged state thereby effecting a shift in gears. Typically, an automatic transmission has several brake band devices, each brake band device corresponding to a specific gear speed for torque transmission.
For reducing loads applied to the brake band during the above engagement operation, the brake band is configured so that it can brake and release of the clutch drum in response to slight changes of the inner diameter of the brake band. Therefore, only a small annular gap is formed between the brake band and the clutch drum in a released or dis-engaged state.
Since the clutch device and the brake band device are used in the wet conditions as described above, a film of lubricating oil is formed over the outer peripheral surface of the clutch drum. Since only the small annular gap is present between the brake band and the clutch drum, the gap is typically filled with the lubricating oil. Therefore, the brake band cannot be quickly released from the clutch drum due to a viscous resistance of the lubricating oil even when an operation is performed to release the clutch drum by loosening the brake band in a releasing or dis-engaging operation. Thus, the brake band impedes the rotation of the clutch drum, and a so-called "drag" occurs. Such drag is undesirable.
In the a dis-engaged state, forces act on the brake band (the strap) causing it to further loosen or enlarge. Such forces result from rotation of the clutch drum.
Tightening and loosening of the brake band (strap) is effected by movement of a hydraulically controlled piston that engages one end of the brake band. One way to prevent drag is to increase the overall movement of the piston, thereby further loosening the brake band. However, when the piston is hydraulically moved to engage the brake band with the clutch drum to brake rotation of the clutch drum, the increase in overall movement of the piston requires an increased use of oil which must be taken from within the automatic transmission. Such an increase of oil may deprive clutch disks of required lubrication. Further, the movement of the piston will take longer than is desirable, possibly affecting performance of the automatic transmission.